Physiological Pharmaceutics

The stomach 77Figure 5.2 Structure of the stomach wallThe stomach adapts to increasing volumes of food by receptive relaxation, whichallows the stomach to expand with little variation in intragastric pressure. The distal orantral portion of the stomach has a wall composed of thicker muscle and is concerned withregulation of emptying of solids by contraction, and by acting as a gastric homogenizer andgrinder. It is co-ordinated with the body in the propulsion of gastric contents towards thepylorus. The pyloric sphincter has two functions. It sieves the chyme and prevents largeparticles of food from being emptied from the stomach before solid masses have beensufficiently reduced in size. It also prevents reflux of duodenal material containing bile andpancreatic enzymes which may damage the gastric mucosa.MucosaThe mucosa of the stomach (Figure 5.2) is thick, vascular and glandular and is thrown intonumerous folds or rugae, which for the most part run in the longitudinal direction, andflatten out when distended. The mucosal surface of the stomach is lined by a single layerof simple columnar epithelium, 20 to 40 µm in height. Approximately 3.5 million gastricpits (foveolae) puncture the lining, each of which serves approximately 4 gastric glands(Figure 5.3). The distribution of gastric glands varies throughout the stomach. The firstregion, 1.5 to 3 cm in length, around the gastric cardia or gastro-oesophageal junction,contains the cardiac glands. The second region, the fundus and body, contains the acidsecretingglands. The third region, which contains the pyloric or antral glands, includes thepylorus and extends past the antrum to the lesser and greater curvatures.Gastric mucusThe surface of the mucosa is always covered by a layer of thick tenacious mucus that issecreted by the columnar cells of the epithelium. Gastric mucus is a glycoprotein whichlubricates food masses, facilitating movement within the stomach, and forms a protective